msm: pm-8x60: Do not default to WFI when not enabled

Return MSM_PM_SLEEP_MODE_NOT_SELECTED in msm_pm_idle_prepare as the
default mode instead of WFI. Skip enter low power mode when this
mode is selected.

Change-Id: Id4a07ba1353b358726c81ed441e0327a9154de8a
Signed-off-by: Priyanka Mathur <pmathur@codeaurora.org>